Bonus items are the three pigs you have to collect on each level. So you just need to destroy a pig after the safe.
This will come very naturally on level 1-3 -- just draw powder from each pig's keg to the safe's keg, then from the safe to the fire.
Note that the level ends automatically a few seconds after the safe is destroyed so the pig needs to be destroyed fairly quickly after you destroy the safe. You also can't draw any more powder after the safe is destroyed so make sure your powder trails are in place.